High Dynamic Range (HDR) Imaging
The amount of variation of light in the world is huge. Normal objects in sunlight and in a shadow often differ in brightness by a factor of 10,000 or more. Objects deep in a room, seen through a small window from outside, can be very dark compared to the outside wall of the house illuminated by direct sunlight. Such environments are difficult to capture, for example, in 8-bit images, which provide a pixel brightness range of only 0 to 255, or even in 10- or 12-bit images as captured by most conventional digital cameras. Conventional film cameras have a slightly higher, but nonlinear, range. However, conventional film scanning techniques are generally limited to less than 16-bit (for example, 10-bit or 12-bit); thus, digitizing conventional film limits the dynamic range.
High dynamic range imaging (HDRI, or just HDR) allows a greater dynamic range of luminance between light and dark areas of a scene than conventional imaging techniques. An HDR image more accurately represents the wide range of intensity levels found in real scenes. Pixel values of digital HDR images thus require more bits per channel than conventional images. An HDR imaging technique may, for example, use 16-bit or 32-bit floating point numbers for each channel to capture a much wider dynamic range of luminance than is captured using standard imaging techniques.
HDR images may, for example, be generated by capturing multiple images at different exposures (e.g., using different F-stops and/or shutter speeds) with a conventional camera, and then combining the image data from the multiple images into a single HDR image. HDR images are not directly displayable to a display device or printable; the information stored in HDR images corresponds to the physical values of luminance. In contrast, the information stored in standard digital images represents colors that should appear on a display device or a paper print, and has a more limited dynamic range. Thus, to view a scene captured in a HDR image, the HDR image may be converted via some technique that approximates the appearance of high dynamic range in a standard digital image, for example an RGB image.
Tone Mapping
Tone mapping is a technique that may be used in image processing and computer graphics to map a set of colors to another set of colors. For example, tone mapping may be used to approximate the appearance of high dynamic range (HDR) images in media with a more limited dynamic range. Conventional methods of tone mapping HDR images may include decomposing an image into a base layer, detail layer, and color layer. In general, the base layer encodes large-scale variations in contrast, while the detail layer encodes fine changes in contrast (i.e., detail). The base layer is then compressed to the lower dynamic range of the output medium by multiplying all values of the base layer by a scaling factor. The detail layer then added back into the base layer. However, in compressing the base layer, these conventional methods use a single, linear scaling factor that distributes the values across the entire range of the output medium. For example, if the range of the output medium is 0 . . . 255, a scaling factor will be used that distributes the compressed values from the base layer from 0 to 255. However, this conventional technique tends to clip details of the scene.